Method of heat treating alloy steel



Patented" June 12, 1945 10mm srAre PATENT orrica METHOD OF HEAT TREATING ALLOY STEEL John M. Hodge, Pittsburgh, Pa., asslgnor to ca'r- H negie-Illinois Steel Corporation, a corporation of New Jersey No Drawing. Application April 15, 1943,

Serial No. 483,201-

5 Claims. (01. .148-21) peratures below that'of the upper critical temperature in heating, may be decreased by the addition of such alloying elements as nickel, chromium, molybdenum, vanadium, tungsten, manganese, silicon, titanium, boronand others; and the present invention relates more specifically to the heat treatment of steels where this transformation rate has been so decreased. It is also known to those skilled in the art, that steels in which the transformation of the austenite occurs at temperatures below 375 C. before a final tempering at a higher temperature, will exhibit superior ductility and impact properties at a given hardness after such tempering as compared to those in which the transformationtakes place at a higher temperature; and the present invention relates'more specifically to a method of heattreatment which will insure this lower tembe "'re transformation without the danger of cracking the metal which is present in the prior art methods of accomplishing this result.

" It is'also known to those skilled in the art, that when the transformation rate is decreased by alloy' 'additions as mentioned above, this transformation rate will be particularly slow in some range of temperatures between 400 C. and 600 C., or, more specifically, at temperatures between those corresponding to transformation to the metallographic constituent known as pearlite and that known as bainite; and the method of the present invention is dependent upon this fact.

The method of the present invention also is dependent upon the fact which is known to those skilled in the art, that upon continuous coolin of material in heavy sections, a differential of temperature will exist between the surface and the center of the material.

One of the objects of the present inventionis "ta-provide a new method of heat treatment of j y steel in sections of three inches and "above; resulting in improved physical properties throughout the body of the material, for, example, 'a higher degree of ductility, impact strength and toughness-than can be obtained by prior art methods.

A further object of the present invention is to adapt the method of the present invention to the heat treatment of fabricated articles such as armor plate, dies, shafts, gears, tools, rolls. machine parts and the like.

Other objects and advantages of the present invention will become apparent as the description proceeds, and the features of novelty will be set forth in particularity in the appended claims.

The method of the present invention consists of the following general steps:

1. Steel of a suitable composition in sections of three inches or above is heated to. above its upper critical temperature and maintained at such temperature for a suitable period of timefor effecting carbide solution. 1

2. The steel is cooled at a rate suficiently rapid to prevent transformation at temperatures above 500 C. and to establish a temperature differential between the center and surface of the material for a period of time such as to bring the surface of the material to a temperature of 350 C. or below and the center of the material to a temperature of 400 C. to 600 C.

3. The steel is reheated immediately so that it is at a temperature of 450 C. to 550 C. throughout its cross section, and maintained for a short time at this temperaturei j 4. The steel is cooled rapidly to a temperature of 350 C. or below.

5. The steeliis tempered at such a temperature as will relieve the strains and result in the desired hardness and physical properties.

The improved method of the present invention is based upon the facts that rapid cooling of heavy sections results in a temperature differential' between the center and outside, that the austenite of alloy steels will transform very slowly in the temperature range of 450 C. to 600 C. and quite rapidly attemperatures of 350 C. and below, and that steels inwhich the austenite transformsat a temperature of 350 C. and below will exhibit superior physical properties after temv perlng.

as described above, upon the first rapid cooling from above the critical temperature the surface of the material which cools to below 350 C. will transform rapidly to martensite. The central portion of the cross section which is permitted to cool only from 400 C. to 600 C. will not transform because of the slow transformation rate in this temperature range, and transformation still will not occur upon rapid reheating to bring the entire cross section to thetemperature range of from 450 C. to 600 C. This reheating tempers the martensite formed in the surface portion during the first rapid cooling and this portion thus is relatively soft. The material then is rapidly cooled again so that the entire cross section articles such as armor plate, dies, shafts, rolls,

tools, bearings, machine parts and the like, and it is illustrated by the following specific example which illustrates the process of the present invention as applied to armor plate:

.An armor plate of the approximate composition carbon 0.35%, manganese 0.25%, nickel 4.00%, chromium 2.00% in the thickness of 17.5 inches was treated as follows:

1. Heated to 800 C. and maintained for four hours.

2. Water sprayed forty minutes. Inserted thermocouples showed the center of the plate to be 550 C. and the surface at 100 C. after this quench.

of not more than about 350 C. with the sections at their centers at a temperature of from about 400 C. to about 600 C., reheating rapidly until the said sections reach a uniform temperature of from about 450 C. to about 600 C. throughout their entire extent, maintaining the sections at this temperature until transformed portions are tempered, cooling rapidly until the center of the sections reach a temperature of not more than 350 C., and reheating the sections to a temperature sufficient to temper the material to the desired hardness and relieve strains.

2. The method of heat treating ferritic steels containing sufllcient alloying elements to suitably retard thermal transformations and to bring such transformations within predetermined temperature ranges, the said steels being in thickness of three inches and above, which comprises heating the plate to a temperature above the upper critical temperature of the steel, cooling the plate from above the upper critical temperature sufflciently s. Reheated to 540 0. in six hours andmain- 3 Tensile Elongation fg";

P. a. i. P. a. i. Per cent Per cent A fracture test of the cross section of the plate showed a completely fibrous fracture with no indication of crystallinity'or brittleness.

The above general method may be adapted to the heat treatment of armor plate in thicknesses of three inches and above, assuming a knowledge of the transformation rate characteristics and the cooling rate of the cross section involved by suitable variations of cooling times and temperatures.

While there has been disclosed as a specific embodiment of the invention the adaptation of the same to the heat treatment of armor plate, it will be understood that the improved process of the present invention is of general application to the heat treatment of alloy steel products in heavy sections, such as dies, shafts, gears, rolls, tools, machine parts and the like, and modifications of the steps of the present invention may be made as may be necessary to adapt the invention to varying conditions and uses, as defined by the scope of the appended claims.

I claim:

1. The method of heat treating ferritic steels containing sufficient alloying elements to suitably retard thermal transformations and to bring such transformations within predetermined temperature ranges, the said steels being in heavy sections, which comprises heating the sections to a temperature above the upper critical temperature, cooling the said sections from above the upper critical temperature at a rate sufficiently rapid to prevent transformation at temperatures above 500 C. and to establish a differential in temperature between the center and surface of the sections for a sufllcient period of time tov cause the rapidly to prevent transformation at temperatures above 500 C. and to establish a differential in temperature between center and surface regions of the plate until the surface reaches a temperature of not more than approximately 350 C. and the center of the plate to a temperature of from approximately 400 C. to approximately 600 C., reheating rapidly to a temperature of from approximately 450 C. to approximately 600 C. throughout the plate, maintaining the said temperature until the surface portions of the plate are tempered, cooling rapidly until the center of the plate reaches a temperature of not more than about 350 C., and reheating the plate to a and to bring such transformations within predetermined temperature ranges, which comprises heating the shape to a temperature above its critical temperature to form austenite, maintaining the said temperature until carbide solution is substantially complete, cooling the steel from the said temperature sufficiently rapidly to prevent transformation of the austenite to pearlite and for such a period of time as will permit the austenite in the surface sections of the shape to transform to bainite and martensite and will maintain the central portions of the shape at temperatures below that of the pearlite transformation and above that of the bainite transformation, then reheating rapidly so that the entire section of the shape is brought to a temperature sufficiently high to temper the transformed portion of the austenite, but below the temperature of transformation to pearlite, rapidly cooling the steel so that its temperature throughout becomes within the range for transformation to bainite or martensite, and tempering the resulting shape at a temperature at which transformation strains are relieved and desired hardness and physical properties'imparted to the steel.

4. The method of heat treating heavy shapes -of ferritic steels containing sufficient alloying elements to suitably retard thermal transformations and to bring such transformations within predetermined temperature ranges, which comprises heating a shape to a temperature above its upper critical temperature until the steel is converted into austenite, then rapidly cooling the said shape while effecting a temperature differential between surface and center portions of the shape suflicient sections at their surface to reach a temperature to transform the surface portions of the steel while maintaining central portions of the shape I into martensite while leaving the central portions unaltered, then reheating the shape sufliciently to temper the martensite, then rapidly cooling the Ill at a temperature between 400 C. and 600 C., thereby eflecting transformation of the surface of the shape into martensite whileretaining the central portions of the shape unaltered, then rapidly reheating the shape to a temperature throughout of from approximately 450 C. to 600 0., thereby tempering the nartensite, then rapidly cooling the shape to a temperature throughout at which the central portions of austenite are transformed intovmartensite as well as the sur-.

face of the shape, and finally reheating the shape to a tempering temperature for relieving strains set up incident to the transformation oi the austenite into martensite.

. JOHN M. HODGE. 

